Find some inexpensive local things to do. Germany, for example, has great public pools. Spend an afternoon at one. Many places have boats you can rent to go out on a lake. If there are any festivals going on, plan your itinerary to include some.

"Does anyone have any suggestions on some good day trips(or overnight) to see out of these cities?"

Guidebooks are good for this purpose... you guys know best what might interest you.

But here are a few typical/quick/easy day trips:

From A'dam: Zaanse Schans - https://www.dezaanseschans.nl/en/

From Cologne: Remagen and its WW II museum -
http://www.bruecke-remagen.de/index_en.htm

http://www.stadt-remagen.de/Tourismu...Stadtrundgang/

More:

From Stuttgart: Esslingen; Ludwigsburg Palace:

http://www.schloss-ludwigsburg.de/en/home/

Black Forest: Perfect place for a group on a budget that wants to do outings. DEFINITELY stay in a small town here. Any hotel, in or hostel in small towns like Gengenbach will give you a free train pass for the whole group (KONUS card) which you cannot get in cities like Freiburg or Baden-Baden.

Gengenbach: http://www.stadthotel-gengenbach.de/...innenstadt.jpg

Day trip to the cities of Freiburg or Basel Switzerland for free. Ride the Black Forest railway to Triberg and other towns for free:

https://www.black-forest-travel.com/...t-railway.html

Day trip to Gutach and the Black Forest open-air museum for free:

http://www.bensbauernhof.com/gutachimschwarzwald.html

KONUS card information:
http://www.camping-muenstertal.de/en/konus-flyer.pdf

(Strasbourg FR is also a good day trip from Gengenbach; the KONUS card gets you to the French border, then it's a few Euros more to Strasbourg.)

Thanks for all the help! I’m going to go buy some guide books and get back to you guys when we’ve for sure decided on our route.

Resist temptation to try to see all of Europe in 30 days and pick destinations that are fairly close together.

I used to travel always with a backpack, but I've switched to a small wheeled suitcase. Here are the things I hate about backpacks:

* You have to take them off when you sit down and put them back on when you stand up.

* In crowded places, you risk bopping people in the face if you turn even slightly. People will look daggers at you, or maybe even call you names in a language you don't understand, especially if there are several of you, unless you take the backpacks off and hold them below your knees.

* You usually have to take everything out to find something.

* Clothing gets more wrinkled in a backpack.

My favorite luggage now is a small (very small) wheeled bag, with an even smaller duffel or daypack piggybacked on top of it.

Three or four people traveling together often spend less by sharing a room in a budget hotel rather than paying for beds in a hostel. A hotel room also offers more privacy, and is more likely to have a private bathroom. Triple and quad rooms are fairly easy to find.

Hostels do have their own advantages. Especially for people traveling alone, they're a great way to meet people, or find someone to share an excursion with. They often have a shared kitchen and laundry facilities. They often have bulletin boards advertising concerts or other activities.

Yes but I highly encourage staying in hostels and more client-oriented youth hotels that also have private rooms (but communal showers, WC- like they will have in college).

Reason - hotels are isolating and in hostels you meet folks from all over the world your age - may go out with them - even travel with them - guys and gals too!

sorry did not read bvlenci's 2nd paragraph in post above mine - said much of what I said. Sorry for my negligence. Again Let's Go Europe is the guidebook to have along for all the dope on hostels and youth hotels and cheap accommodations.

Get FIRM commitments from your friends that they are ABSOLUTELY, POSITIVELY going to take the trip.

Nothing wrong with going alone however as I did at that age and found youth hostels and trains great places to meet others my age and traveled with off and on.

If they don't come send them postcards.

I've been using backpacks for 25 years, and never once hit anyone in the face by turning wildly or even slightly - and oh how I have tried! Locals are quicker than you think. However, TWICE I have been tripped up in an airport or train station by an inconsiderate person suddenly cutting across my path and dragging their wheeled suitcase behind them. A person's overall footprint while walking is larger with a wheeled suitcase than with a similar-sized backpack.

I've never had to take everything out to find something. Plan ahead and put the go-to items in the easy to reach pouches on the outside. A wheelie wouldn't solve that problem regardless.

Yes, I have to remove my backpack to sit. It's a small price to pay to walk with my luggage with both hands free.

I've never noticed clothing getting more wrinkled in a backpack than a regular suitcase, but my clothes consist mostly of underwear, socks, and t-shirts. Some clothing items will need a good ironing anyway once I get to my room. A wheelie would not change that.

My soft-sided backpack is much easier to fit into an overhead compartment than any wheelie could ever hope to be. The soft-sides let it form-fit to an extent into whatever space might be left up there - which is very considerate of me because it leaves more room for hard-sided cases like wheelies. You're welcome.

With a backpack I don't need to fret about cobblestone streets in Paris, or a wheel breaking off thereby transforming my wheelie suitcase into a cinder block.

Thus, backpack > wheelie, QED. Still, backpacks aren't for everyone. If I had a bad back or a bum shoulder then I would switch to a wheelie.

I also use a small backpack. I rarely put both arms through it unless I will be walking a long distance.

You already got some great suggestions. Just a few additions:

Switzerland: Fully agree that you should drop this, unless any of you have a strong wish to go there

Germany: Everyone always seems to love Berlin. It also tends to be a bit cheaper than other German cities. A day trip might be Potsdam.

Spain: Agree that for younger people, Barcelona tends to have more appeal than Madrid. It's quite difficult to find accommodation in Barcelona - so if you want a nice hostel, you should definitely plan this in advance and not think that you can just wander around finding a place.

Belgium: Are you sure it's Brussels you wish to see? Bruges and Ghent might be much more interesting, easily reached by train from Brussels. Both quite small - Bruges is a bit prettier I think, but also more overrun by tourists. Ghent is also very pretty and has a more studenty feel to it. Watch the movie "In Bruges" to get an idea what to expect

Amsterdam: Great idea, you should definitely go there. There also tend to be good international flight options from Amsterdam, so this might be a good place for the beginning or end of the trip. Also here I think it's quite difficult to find accommodation, this should be planned in advance.

London: As someone said, this also seems to have good international flight options. Might be a nice place for the first few days to get over the jetlag, as it'll all be in English and therefore pretty straightforward.

Prague: You didn't add any places in the East, which tend to be cheaper. Maybe add Prague?

Italy: This should be on everyone's Europe itinerary I think. Not sure about which cities though. I personally loved Venice. Haven't been to Rome yet but people seem to like it. Florence is nice, but it's a lot about art. After a few days in Florence I went to Bologna, which I preferred even though it doesn't have the main sights as Florence - it just had a nicer vibe, with lots of students hanging around. So as others have said, think about what you want to do - if you're into art, Florence is great. If not, it's still nice, but might not be worth a complicated de-tour.

While I agree that it's better to not put too much into a travel itinerary, I also think that this is something that you learn as you travel. So if in your first trip to Europe you want to see lots of different countries, plan sensibly and take the travel time into consideration, but still do as you wish (partly ignoring the advice of everyone with decades more experience - which might lead to you being tired and exhausted, but means you're not going home wishing that you'd stuck to your original plan).
e.g. in Paris, I think you should have 4 full days. BUT, if you can't manage this, if you only have one day and spend this day wandering around the Montmartre area, discovering some small alleyways and drinking wine in a bistro, I think you'll be very happy with your time in Paris.
If you assume 5 days per city (time spent there will be lower due to travel time), this means you can cover around 6 destinations in 30 days.

Enjoy your planning, I'm sure it'll be a great trip.

Th main thing about sharing rooms is that I’ve found people drop out of trips for all kinds of reasons, sometimes at the last moment. And then that shared room is a problem for the rest. Even if you aren’t terribly budget conscious, there’s a good chance one of your travelling companions are and would be very upset if they found out they had to pay even $5-10 more.

This situation has happened a few times to me. So now if my friends want to go somewhere, I steer them towards hostels. That way, if someone cancels, it doesn’t affect others. This is less of an issue now that we’re older- but in college this was a big problem.

I’ve dragged my bag everywhere and haven’t had a wheel break yet...and have been hit by someone’s backpack on the bus. I don’t think my carry on is bulkier than a backpack. Fits almost everywhere overhead.

Maybe (just maybe) you don't realize when its happened. I've been hit by packs more than a few times on the Metro, the Tube and in airplane aisles. Not all the time because I am ALWAYS on alert for backpacks at my head level. I'm 5' tall and once you've been clocked in the head a few times you learn to be vigilant. Planes are actually worse than the Tube if you are sitting in an aisle seat. People must forget how narrow the aisle is because they will turn to check the seat numbers and stick their backpack right in your face. But again you need to decide yourself is a backpack or rollaboard is better for you.

Heavier backpacks are more unwieldy to manouver thru crowds or train cars and to lift into overhead luggage racks. I used backpacks for years until backpack with wheels can out - the best of both worlds and never would go without something that I can wheel around - often easily walking miles with rolling backs.

Worst case I've seen of too heavy backpacks - young 20s type in Nice train station when trying to get backpack on she fell over backwards! If really light backpacks can be fine but with wheels weight is no real problem.